In a conventional steering system such as for outboard motors used on boats, the propulsion unit, typically mounted on the transom of the boat, is pivoted about a vertical steering axis upon steering actuation by the operator at the helm. One typical steering system for a boat having a transom mounted engine comprises a steering cable extending between the steering helm and the propulsion unit so that steering at the helm actuates the cable for causing steering movement of the propulsion unit about a steering axis. A conventional steering cable is the push-pull cable comprising a reciprocative inner core slidable in a protective, flexible outer sheath or housing. One end of the cable is actuably connected to the steering helm, and the other end is actuably connected to the steering mechanism of the propulsion unit. When the wheel is turned at the helm, the cable is actuated by a push-pull movement of the inner core, thereby causing a steering movement of the propulsion unit. Hydraulic activated steering means can be used in place of the cable steering, wherein hydraulic fluid, e.g. oil, is pumped from the steering helm through conduits to a cylinder-piston control means in response to rotation of the steering wheel in one direction or the other. Actuation of the control means actuates the steering mechanism of the propulsion unit, thereby turning the propulsion unit in a common direction. Prior art teaching steering systems of this type include the following U.S. Pat. Nos.: 4,592,732; 4,615,290; 4,632,049; 4,568,292; 4,295,833; and 5,074,193; and French patent 1,133,061. Additionally, British Patent Application 2,159,483A discloses a power steering system for an outboard having a hydraulic cylinder-piston assembly and a control valve which is operated by an actuator including a push-pull cable to selectively extend and retract the piston rod and effect steering of the propulsion unit. The power steering assist system as taught in each of the prior art patents and British application identified above, however, is mounted onto and supported by the propulsion unit. Mounting the power steering system on the propulsion unit is disadvantageous for a number of reasons. First, the propulsion unit mounting position must be changed because there is a steering apparatus to conflict with the boat transom design during vertical tilt movement. In order to mount the power steering system to the propulsion unit, special bracketry is required for each engine design, because the mounting pads vary markedly depending on the design. Exemplary of engine mounting is the disclosure in the above identified British Patent Application, where, as shown in FIG. 7, the power assist unit 120 is mounted on a propulsion unit 10, which is mounted to a boat transom 22. As the propulsion unit 10 tilts about the horizontal axis 42, the power assist unit 120 may come into contact with the boat transom, thereby limiting its applicability. A second disadvantage is that the power steering system, including the supply and return lines which are under high pressure, are subjected to sun rays, salt water corrosion, and physical abuse because of exposure. Thirdly, such designs as shown in the prior art do not allow for steering shock to be absorbed partially by the steering cable, in that any steering shock is prevented from passing beyond the power assist steering system causing a high stress on the propulsion unit steering components. Lastly, the systems of the prior art, and in particular such a system as taught by the aforesaid British Patent Application, are designed to continuously supply fluid to the system, and not just when steering movement occurs. This constant fluid supply system wastes propulsion engine horsepower.
U.S. Pat. No. 497,706 discloses an in-line steering assist system in that the system is mounted remote from the rudder disposed adjacent the propeller. A retractable carriage is moved by a fluid actuated piston, and a cable extending from the piston, around pulleys on the carriage and to the rudder disk, moves the disk in response to movement of the piston. Little or no torque created at the rudder is consumed by the steering assist means to thereby reduce the steering effort required at the helm.
The prior art also discloses a steering mechanism for a boat utilizing a gear assembly for steering the boat, typically the gear assembly actuates a steering member on the engine in response to the steering input at the steering wheel. This prior art includes U.S. Pat. Nos.: 1,425,887; 1,852,151; 2,700,358; 2,891,498; 2,939,417; 3,181,491; 3,669,146; 4,416,637; 4,890,683; and 5,018,469. None of these references, however, incorporate a power steering assist mechanism with the gear assembly.
Power steering systems for marine vehicles mounted remote from the propulsion unit, and overcoming the several disadvantages of the prior art, are disclosed in U.S. Pat. No. 5,228,405, and in co-pending U.S. patent application Ser. No. 08/012,552, both of which are assigned to the same assignee as the subject application, and are incorporated by reference into this specification. In such hydraulic power steering systems, the torque originating from the propulsion unit is overcome, thereby restraining the steering forces created by this torque. That is, the power steering assist means reduces the effort at the steering helm or wheel to only the effort required to operate the hydraulic assembly, which is independent of the torque generated by the propulsion unit. The power steering system of the issued patent comprises a push-pull cable as the input steering member operably connected to a hydraulic cylinder-piston assembly having a valve actuating means, and an output steering member such as a push-pull cable operably connected to the cylinder-piston assembly and to the steering member of the propulsion unit. Steering at the helm effects a common movement of the steering arm. In the pending application, the input and output steering members comprises a gear drive means.
Power steering assist for automobiles is commonplace, which basically employs a hydraulic power cylinder and piston, such as shown in U.S. Pat. No. 3,121,345 to Zeigler et al. In Zeigler et al., the power steering system disclosed in FIG. 4 comprises a cylinder 10' divided by piston 94 to form chambers 96 and 101, a rotary valve 84, and input shaft or stub shaft 90 connected to the steering shaft. When the steering wheel is turned, the input shaft rotates the valve to open fluid passageways, and oil is directed to the proper side of the piston-chamber and is released from the opposite chamber (depending on the direction of turn), thereby actuating sleeve 40' and shaft 54' to rock pitman arm 60'. The recirculating ball system permits linear travel of the sleeve, which is translated into rotary movement of the shaft. A torsion bar 86 maintains the valve in a neutral position, and turning the steering wheel deflects the torsion bar to change the relationship between the fluid passageways. When in a neutral or no-steering position, the torsion bar maintains the relative positions of the fluid passageways so that fluid is delivered under equal pressure to both chambers in the cylinder. Thus, when the steering action is completed, the power steering assist is returned to a neutral or center position, which is characteristic of power steering for automobiles because it is essential to have feed back from the road so that a sense of steering is retained. For example, if the car hits a small obstacle or a front tire goes flat, this load or force is immediately fed back to the steering wheel. This load is loosely analogous to the torque created by the propulsion unit of a boat, but for a marine vehicle there should be essentially no feed back to the steering wheel.
Additional references relating to steering systems for land vehicles include U.S. Pat. No. 3,473,324 to Mercier. This patent discloses a distributor valve or spool valve 36 and actuator 35. Turning the wheel 12 directs oil to one side of the valve, and the oil pressure laterally moves member 61 (comparable to a piston) which opens the passageways to force oil into the actuator thereby actuating piston 47 and moving rod 48. U.S. Pat. No. 4,316,519 to Taig discloses a power steering unit comprising a spool valve 142 (see FIGS. 7 and 8), which is actuated on steering thereby directing fluid to the cylinder 66 and against the piston 56 or 62 to move the cylinder sleeve and turn the wheels. There is disclosed in U.S. Pat. No. 2,094,466 to Proctor a hydraulic system comprising a cam actuated valve with check valves to direct the flow of fluid and actuate a jack. Lastly, U.S. Pat. No. 5,289,893 to Yamamoto et al. discloses a steering control mechanism comprising a rack 84 and pinion 92 for actuating lateral shaft 82 linked to the actuator 130 for steering the wheels, and a three-way solenoid valve 13 for directing fluid to either side of the piston 134 in the actuator.